1. Field of the Invention
This invention relates to a valve means, which adjustably controls the flow direction and the volume flow so that this is substantially constant for each setting and independent of load variations, and where the adjusting can be carried out mechanically by positioning, electrically by a force or positioning, or hydraulically or pneumatically by a pressure, but where the control work can be held so low that one person, for example by a lever, or alternatively a relatively small electromagnet, can control the valve means directly and without auxiliary force. The volume flow passes alternatively, depending on two possible cases, in one case from a pressure source to two alternative sides of, for example, a hydraulic cylinder, and in the other case from one of the two sides of the cylinder to a return conduit, in such a manner that the requirements of low valve leakage and safety in the event of conduit rupture cn be satisfied simultaneously.
2. Description of the Prior Art
Hydraulic directional valves have the object to control the direction of movement and the speed of hydraulic cylinders and rotary motors. The directional valves most commonly used control adjustably the speed, i.e. in this case the volume flow, via adjustable throttlings in the valve. With these valves, the volume flow depends on the size of the load and on the load variations. In order to decrease or eliminate the changes in speed caused by variations in the load, in recent years valves have come into use which are load-independent. These valves maintain at every setting a volume flow independent of the pressure drop through the valve.
Basically, there exist to-day two known principles meeting the requirement of controlling the volume so as to be substantially constant and independent of the pressure drop through the valve.
The oldest principle, most known and applied, is based on two co-operating valves connected in series. One of these co-operating valves controls adjustably the volume flow by adjusting the throttling area through the valve. Thereby, the pressure drop through the throttling area always is maintained constant, and thereby the volume flow passing through the valve for each setting of the throttling area is substantially constant and independent of the pressure drop prevailing through both valves together. The second co-operating valve has the object to automatically throttle the volume flow so that the pressure drop through the adjustable throttling place of the first-mentioned valve is relatively low and most important for a constant pressure drop.
The newest principle, which so far has not been used to a great extent, is based on the capability by one and the same valve function to control a volume flow, which is adjustable, and to maintain for each setting a substantially constant volume flow, which is independent of the pressure drop through the valve. At varying pressure drops through the valve, thus, the prevailing throttling area must vary in such a manner, that the volume flow remains constant for every valve setting. This is effected by so designing the valve means that flow forces are obtained which close the throttling area, and that simultaneously an opening constant adjusting force acts on the valve means, which force determines the volume flow to pass through the valve, and also a non-adjustable spring force from a centering spring acts in the closing direction on the valve means. The throttling area must increase linearly with the movement of the valve means in the respective opening direction. The constant adjustable opening force to-day is brought about hydraulically by causing an adjustable pressure to act on an area, for example the end area of the valve slide.
As regards adjusting the flow direction and the size of the volume flow, two types of application fields can be distinguished. Especially in mobile working machines such as an excavator, loader, hoisting crane etc., the different movements of the machine are controlled by a driver. Especially in industrial manufacturing processes the machine is controlled by electric or, exceptionally, pneumatic control systems. The members attending to the primary adjusting work are the arms and hands of the driver or small electromagnets. The adjusting work within the capacity of these members renders it possible to control only the smallest valves of the respective type, and mostly with only small margins. As soon as the valve size increases, or the adjusting work is desired to be limited, a so-called servo control must be applied. This implies normally, that primarily a small hydraulic valve of pressure reducing type is controlled, which in its turn via pressure controls the main valve and the size of the volume flow. Hereby, of course, the system becomes more expensive and complex.
A directional valve, in addition to controlling the direction and volume flow, has the object to be capable to hold the load motionless, i.e. to efficiently seal hydraulic motors and cylinders both under normal conditions and also from a safety aspect in the event of conduit rupture. This implies that the systems must be designed so as to have a correct system structure, i.e. the components must be positioned correctly, and each component must have the correct function. To-day, in principle two hydraulic systems are designed which have the alternative system structures as follows.
The valves are combined in one block, which is located relatively centrally relative to cylinders and motors. Two high-pressure conduits connect every cylinder or motor to the central block. This structure normally is called central structure. It does not ensure safety at conduit rupture.
The valves are assembled with the respective cylinder or motor. Two main conduits, one for high pressure and supply, and one for low pressure and discharge, are drawn so that every cylinder or motor and its valve are connected. This system normally is called motor structure. This structure can ensure safety at conduit rupture and also efficient sealing of the cylinder or motor.
The valves are assembled in a central block, and every cylinder or motor, in addition, is provided with a valve. Two high-pressure conduits for every function are drawn between the central block and the respective valve on the cylinder or motor. It can here be imagined in principle to design a separate return conduit as a main conduit. This structure is called mixed structure. It can ensure safety in the event of conduit rupture and also an efficient sealing of cylinder or motor.
It can be stated that manual control without servo control practically is possible only with central or mixed structure. The driver must be near the valve. It also is apparent that the safety requirement in the event of conduit rupture can be met only when there is a valve function on the cylinder or motor. It further can be stated that manual control without servo control with the requirement of safety in the event of conduit rupture can be satisfied only with mixed structure. In electric control, without servo control and simultaneously with the requirement of safety in the event of conduit rupture being provided, this can be satisfied both with motor structure and mixed structure. It is here to be distinguished that the requirement of tightness must be met by the valve located on the cylinder or structure, at the same time as the central block in the mixed structure can be loose. The control of the volume flow from the motor or the two pressure sides of the cylinder must occur with the requirement of tightness, while at the mixed structure, and at times also at the motor structure, the requirement of tightness at the control of the volume flow from the pressure source to the two pressure sides is low and dictated by other respects. The sealing between the valve on the motor and the pressure side of the main conduit or the central block is effected by check valves, which simultaneously ensure tightness and safety in the event of conduit rupture.
Commercially available to-day are manually directly controlled valves with mixed structure and volume flow control based on two co-operating valves. The principle of volume flow control via one single valve function is realized only with servo control. This can be explained as follows. In control with two co-operating valves, the driver must overcome only the friction and a weak friction-overcoming return spring at the adjusting of the valve function, which adjusts a certain adjustable throttling area. In the other type with only one valve means, the valve moves in pace with the load variations. It is impossible to control directly on the valve means, because the driver's hand preferably must be motionless, independent of the slide position and dependent only on the selected position of adjustment. It is equally disqualifying, that the adjusting force due to acceptable control accuracy must be set at least ten times higher than the friction force. As the friction force per se already at small valves requires an adjusting work, which is close to the limit permissible at direct control manually or by small electromagnets, it is obvious that the principle with one valve means cannot be applied with the present state of art.
In view of valve volume, weight and cost, the principle with one valve means is most advantageous, which can be utilized in systems with servo control. In most systems it is desired, for cost reasons, to avoid servo control, if possible. Therefore, the principle with two valve means is to-day still the only possible solution.